1. Field of the Invention
The present invention relates to a method of producing grain-oriented silicon steel sheets having excellent magnetic properties.
2. Description of the Prior Art
Grain-oriented silicon steel sheets are mainly used in an iron core of a transformer and other electric instruments, and are demanded to have excellent magnetic properties, that is, have an excellent magnetizing property and a low iron loss. Recently, technics for producing silicon steel sheet have been progressed; and a grain-oriented silicon steel sheet having an excellent magnetizing property, that is, having a high magnetic induction of B.sub.10 value of more than 1.89 T (teslas) has been obtained and contributes to the production of small size transformer and other electric instruments and to the decreasing of noise; and further a grain-oriented silicon steel sheet having a low iron loss of W.sub.17/50 .ltoreq.1.10 W/kg in a sheet thickness of 0.30 mm, that is, having an iron loss of not more than 1.10 W per kg of the steel sheet when the steel sheet having a sheet thickness of 0.30 mm is magnetized under a magnetic induction of 1.7 T and at a frequency of 50 Hz, has been obtained.
A fundamental requirement for obtaining a grain-oriented silicon steel sheet having such excellent magnetic properties is that secondary recrystallized grains having (110)[001] orientation are fully developed during the final annealing. It is commonly known that the following conditions are required for this purpose, that is, the presence of inhibitor which suppresses strongly the growth of primary recrystallized grains having an undesirable orientation other than the (110)[001] orientation during the secondary recrystallization, and the formation of recrystallization texture which is effective for the predominant and sufficient development of secondary recrystallized grains having a strong (110)[001] orientation. As the inhibitors, there are generally used fine precipitates of MnS, MnSe, AlN and the like. Further, grain boundary segregation elements, such as Sb, As, Bi, Pb, Sn and the like, are occasionally used together with the inhibitor to enhance its effect. In order to form the effective recrystallization texture, a method wherein the hot rolling condition and the cold rolling condition are properly combined, is carried out, and a complicated step which consists of two cold rollings with an intermediate annealing between them, is carried out for this purpose.
While, a slab to be used as a starting material for the production of grain-oriented silicaon steel sheet has hitherto been produced from molten steel through ingot making and slabbing, but is recently produced directly from molten steel by the continuous casting. The defects in the crystal texture and recrystallization texture due to the use of the continuously cast slab causes troubles in the grain-oriented silicon steel sheet product. That is, when it is intended to obtain fine precipitates of MnS, MnSe, AlN and the like, which are effective as an inhibitor, it is necessary that a slab is heated at a high temperature of not lower than 1,250.degree. C. for a long period of time before the hot rolling to dissociate and to solid solve fully the inhibitor element into the steel, and the cooling step at the hot rolling is controlled to precipitate the inhibitor element having a proper fine size. However, in the continuously cast slab, extraordinarily coarse crystal grains are apt to develop during the high temperature heating of the slab as described above, and incompletely developed secondary recrystallized texture called as fine grain streak is formed in the resulting silicon steel sheet product due to the extraordinarily coarse crystal grains, and the silicon steel sheet product is poor in the magnetic properties.
There have higherto been proposed several methods in order to prevent the formation of the above-described fine grain streak and to improve the magnetic properties. For example, Japanese Patent Laid-Open Application No. 119,126/80 discloses a method, wherein a slab is subjected to a recrystallization rolling when the slab is hot rolled into a given thickness, that is, the texture of the slab just before the recrystallization rolling is controlled such that .alpha.-phase matrix contains at least 3% of precipitated .gamma.-phase iron, and the slab is subjected to a recrystallization rolling at a high reduction rate of not less than 30% per one pass within the temperature range of 1,230.degree.-960.degree. C. The inventors have proposed in Japanese Patent Application No. 31,510/81 a method, wherein a slab is mixed with a necessary amount of C depending upon the Si content, and not less than a given amount of .gamma.-phase iron is formed within a specifically limited temperature range during the hot rolling, whereby coarse crystal grains developed in the slab during the heating at high temperature are broken to prevent effectively the formation of fine grain streak in the product.
However, according to the above described method of forming not less than a given amount of .gamma.-phase iron in a slab during its hot rolling, although formation of the fine grain streak in the product can be prevented, the aimed magnetic properties can be not always obtained, and moreover the prevention of the formation of the fine grain streak is very unstable, and fine grain texture may be formed all over the product to deteriorate noticeably its magnetic properties. Therefore, this method is still insufficient in the stability of the effect, which is a most important factor in the commercial production of grain-oriented silicon steel sheet.